Electrodeposition of stress-free metal deposits



United States Patent ()fiti ce 3,338,804 Patented Aug. 29, 1967 ABSTRACT OF THE DISCLOSURE Electroplating baths and processes for electrodeposition of stress-free nickel, cobalt, zinc or copper wherein the stress-reducing additive present in the bath is an alphaketo carboxylic acid having 3 to carbon atoms or the Water soluble salts, esters or amides of said acid and said stress reducing additive is present in a concentration between about 0.25 g./l. and g./l.

This invention relates to metal electroplating baths and to a process for the production of stress-free metal electrodeposits. More particularly it relates to additives that serve as stress-reducing agents in such baths.

Electroplating is widely used for the purpose of altering the surface characteristics of metals and nonmetals so as to provide improved appearance, ability to withstand chemical attack, and improved resistance to tarnish and corrosion or to alter dimensions. In addition it is also used for the direct electro-forming of articles which because of their intricate shapes or requirements for surface finish would be difiicult or costly to produce by conventional metal working procedures.

A number of metals are deposited from electroplating baths in a condition of internal stress which may be either tensile (positive) stress or compressive (negative) stress. This condition may be evidenced, for example, by lack of adhesion or by the cracking or peeling of the metal electroplate. It is particularly disadvantageous in electroforming applications where the products formed may be warped or may have poor dimensional stability as the result of internal stress.

It is known that the amount of internal stress in an electroplate is dependent to a large degree upon such factors as the composition of the bath and its current density, acidity, and temperature and that the stress can be reduced to some extent by the choice of conditions under which the plating is carried out. In addition the presence in the bath of certain materials, for example, primary brighteners such as saccharin, is known to reduce stress in the electrodeposits. In some cases these additives reduce the stress from a high tensile stress to a low tensile stress; in others they reduce a tensile stress to a compressive stress. No agent has been previously reported that will reduce either tensile stress or compressive stress to. zero and maintain it at zero over a wide range of acidity, current density, temperature, and concentration.

' In accordance with the present invention, it has been found that metal electrodeposits having zero internal stress can be prepared by electrolyzing an aqueous solution of at least one water-soluble salt of the metal which contains a small amount of an ob-keto acid having from 3 to 10 carbon atoms or a water-soluble salt, ester, or amide of saidacid. These stress-reducing additives can be used in bothelectroplating baths that ordinarily produce positively-stressed metal deposits, such as Watts baths, and electroplating baths that ordinarily produce negativelystressed metal deposits, such as Zinc sulfate baths, to produce metal deposits that are stress-free over a wide range of acidity, current density, temperature, and concentration.

In addition to being free from internal stress, the metal electrodeposits prepared in accordance with this invention are characterized by excellent hardness, ductility, adhesion, and corrosion resistance.

A Wide variety of a-keto acids may be used as stressreducing additives in the electroplating baths of this invention. These include both aliphatic and aromatic monocarboxylic and dicarboxylic acids having from 3 to 10 carbon atoms. Illustrative of these acids are pyruvic acid, a-ketobutyric acid, u-ketocaproic acid, a-ketocaprylic acid, a-ketopelargonic acid, a-ketocapric acid, ketomalonic acid, ketosuccinic acid, a-ketoglutaric acid, u-ketoadipic acid, and benzoylformic acid. Water-soluble salts, such as the alkali metal, ammonium, alkylamine, alkanolamine, and magnesium salts of these are fully equivalent to the acids as stress-reducing agents and may be used in their place in the various embodiments of the invention. Salts of the a-keto acids that have limited solubility in water, such as the cobalt and nickel salts, may be used in those baths that require only a small amount of the additive or in combination with the more soluble salts. Water-soluble esters and amides as well as those derivatives of the acids that are hydrolyzed during electrolysis to the a-keto acids or their water-soluble salts may also be used. These include the loWer alkyl esters, the lower alkanol esters, lower glycol esters, the unsubstituted amides, and lower alkyl substituted amides, for example, the methyl, ethyl, propyl, butyl, hydroxyethyl, hydroxypropyl, ethylene glycol, and propylene glycol esters and the unsubstituted amide, N-methylamide, N,N-di methylamide, and N,N-dipropylamide.

The stress-reducing additives disclosed herein may be used advantageously in conjunction With many types of acidic and alkaline electroplating baths and especially with electroplating baths for the production of electrodeposits of nickel, cobalt, iron, zinc, copper, cadmium, tin, chromium, and alloys or other mixtures of these metals. These include, for example, Watts baths, nickel sulfamate baths, nickel chloride baths, nickel fiuoborate baths, nickel sulfate baths, copper sulfate baths, copper pyrophosphate baths, copper fluoborate baths, cobalt sulfate baths, cobalt sulfamate baths, zinc sulfate baths, chromic acid baths, and the like. They are of particular value in acidic baths since their use in these baths results in the formation of deposits having an excellent combination of mechanical properties that heretofore could be obtained only when alkaline baths were used.

Only relatively small amounts of the a-keto acids or their water-soluble salts, esters, or amides need be used in the electroplating bath since concentrations as low as 0.25 g. per liter have been found to reduce the internal stress of the metal electrodeposits to zero. In most cases approximately 0.5 g. to 10 g. per liter of the additive is used, with especially advantageous results being obtained from the use of 0.75 g. to 6 g. per liter. As much as 15 g. per liter or more of the oc-kCiO compound can be used, but the use of these larger amounts brings about no particular advantage. The amount of the u-keto compound used in any case is largely dependent upon such factors as the type of electroplating bath being used and the molecular weight and water solubility of the additive.

While these additives will produce stress-free metal electrodeposits when used as the sole additives in electroplating baths, they may also be used in conjunction with brighteners, leveling agents, sequestering agents, inhibitors, Wetting agents, conductivity-promoting salts, porosity-preventing agents, and other additives customarily used to improve metal electrodeposits.

The surface of the objects to be electroplated in accordance with the processes of this invention may be formed of any material upon which electroplates are ordinarily deposited. For example, it may be a metal, such as iron, steel, zinc, copper, aluminum, lead, or a metal '3 e9 4 alloy or a non-metal, such as a synthetic resin, wax Example 1 paper, cloth, wood, plaster, or glass. The use of the electro 1 p ating baths of this inventlon The r f addmves be added to the for the production of stress-free electrodeposits of varielectropizitmg bath In any convelilent For ous metals is shown in Table I. In each case a wellthe additive or an aqueous solution f the additive may 5 known type of electroplating bath was used with and be addefi dlrecfly to the electroplatmg bath an atpnce without the stress-reducing additives in order to demon- Or contitiuously throughou? the p i Alternatwely strate the effect on the internal stress of the electrodethe addmve may be combmed with or Incorporated m posit of the addition of a small amount of the additive.

the metal that acts as the anode and gradually dissolved in the bath during the course of the electroplating process.

A preferred process according to this invention for producing stress-free electrodeposits of a metal selected from the group consisting of nickel cobalt iron Zinc Copper ture of a helix as the helix is electroplated. The stress cadmium, tin, chromium, and mixtures thereof comprises in the deposit causes the helix to Wind more tightly or electrodepositing said metal from an electroplating bath to unwind depending on whather the deposit stress is In these tests as well as in those described in the other examples, internal stress was measured by means of the Brenner-Senderoff spiral contractometer. This instrument indicates stress by a change in the radius of curvacomprising an aqueous Solution of at least one water compressive or tensile. The change in radius of'curva- Soluble Salt of the metal to be deP0S1ted approxl' ture as indicated by the angular displacement of one end mately 8- 2%- P of 531d bflth a of the helix while the other end is held rigid is a measure stress-reducing addltlve whlch 18 n m-keto acld havlng O of the internal stress of the deposit and can be converted from 3 to 10 carbon atoms or a water-soluble salt, ester, 2 into conventional stress units, such as pounds per square or amide of said acid. inch.

TABLE I Cathode Internal Ex. No. Metal Deposited Ingredients of Bath ConcentrapH Tempera- Current Additive Concentra- Stress, tiou, g./1. ture, F. Density, tion, g./l. p.s.i.

amp/ft.

1A-1 Nickel NiSO -fiH O 300 4. 0 140 40 None 21, 300 NiCl -6H O 60 lA-2 Boric Acid 37. 5 4. 0 140 40 PYIUVIC Acid 2. 25 0 1B-1 Cobalt C0SO4-7H2O 450 2. 4 100 40 None 92, 200 4 0001 -61120 113-2 Boric Acid 38 2. 4 100 40 Pyruvic Acid 4. 5 0 1-0 Copper CuSOyfiHzO 188 0. 5 100 100 None 12, 600

H2304 74 16-2 0. 5 100 100 Pymvic Acid 1. 87 0 lD-l Copper CuzPzO 1-3I'I2O 94 8. 7 138 40 None 4, 000 I K4P201 337.5

1D-2 NH4OH 4. 7 8. 7 138 40 Sodium Pyruvate 1. 25 0 112-1 Zinc ZnSO 4-7H2O 350 3. 9 120 100 None 2, 300

(NHilz i 30 113-2 3. 5 120 100 Pyruvic Acid 2. 1 l 0 112-3 3. 9 120 100 Benzoylformic Acid 1. 0 o

The following examples demonstrate the way in which E xample 2 this invention may be practiced. It is to be understood, however, that these examples are not to be construed The effect of the use of the stress-reducing additives as being limitative, but are furnished merely for the on various types of nickel electroplating baths is shown purpose of illustration. in Table II.

TABLE II Ex. Concen- Temper- Cathode Couceu- Internal No. Type of Bath Ingredients of Bath tration, pH ature, Current Additive oration, Stress, g./l. F. Density g./l. psi.

2A-1 Watts Bath NiSO4-6H O 300 4. 0 140 40 None 21, 300

NiCl -GH O 2A-2 Boric Acid 37. 5 4. 0 140 40 Sodium Pyruvate 2. 8 0 2A-8 4.0 140 40 Ammonium Pyruvato 2. 0 0 2A4 4. 0 140 40 Magnesium Pyruvate 2. 1 0 2A-5 4. 0 140 40 'Iriethanoiamine 6. 0 O

Pyruvate 2B-1 Sulfamate Ni(SO NHg) g 330 4. 0 120 None 7, 200 Boric Acid 30 213-2 4.0 100 Sodium Pyruvate 1. 9 0 2B3 4. o 120 40 None 3, 000 213-4 4.0 120 40 Sodium Pyruvate 0. 94 0 213-5 4. 0 120 40 Sodium Pyruvate 1. 9 0 20-1 Chloride NiCl -(iHsO 300 2.0 40 None 64, 200 Borie Acid 38 20-2 2. 0 140 40 Pyruvic Acid 2.1 0 2D-1 Fluoborate Ni(BF4), 300 1.8 l 120 40 None' 30,800 Boric Acid 30 2D-2 1. 8 l 120 40 Sodium Pyruvate 3. 0 0

Example 3 TABLE IV Stress-reducing additives were used 11 Watts baths that Tempera Cathode Cub Internal contained sulfo-oxygen compounds as primary brighteners Ex. No. pH ture, F. rent Density, Stress, p.s.i. alone or in combination with a secondary luster-pro- 5 amp/f ducing brightener. The composition of the baths and the operating conditions employed are given in Table III. :38 fig 5g 8 From the data in this table it will be seen that the baths 4.5 140 20 0 containing a stress-reducing additive produced bright or is? E3 38 8 semi-bright stress-free nickel electrodeposits, whereas 10 3.0 140 40 0 those that did not contain one of these additives proi8 i8 3 duced deposits having high compressive stress. i-g 2g 8 4:5 115 40 0 4.5 115 80 0 M 3.0 115 40 0 4N.. 3.0 115 80 0 TABLE III Concentra- Tempera- Cathode Concentra- Internal Ex. Type of Bath Brightener tion, g./l. pH ture, F. Current Additive tion,g./l. Stress, No. Density p.s.i. 3A-1 Watts Bath 1 Sodium salt of 1. 9 4.0 140 40 None 14, 000

(Semi-bright) saccharin 3A-2 4.0 140 40 Sodium Pytuvate 4. 7 0 3B-1 Watts Bath 1 Sodium salt of 1.12 4. 0 145 40 None -5, 000

(semi-bright) benzaldehyde- 3B-2 o-sulfonic acid 4.0 145- 40 Sodium Pyruvate 1. 4 0 30-1 Watts Bath 1 Sodium salt of 2. 4.0 145 40 None 9, 800

(semi-bright) benzaldehyde- -2 o-sulfonic acid 4.0 145 Sodium Pyruvate 1. 4 0 3D 1 Watts Bath 1 Sodium salt of 3. 37 4. 0 145 40 None -13, 000

(semi-bright) benzaldehyde- 3D-2 o-sulionic acid 4.0 145 40 Sodium Pyruvate 2. 8 0 3E1 Watts Bath 1 Sodium salt of 1. 12 4.0 140 40' None -10, 600

(semi-bright) naphthalene- 3E-2 1, 3, 6-trisul- 4.0 140 40 Sodium Pyruvate 0.9 0

ionic acid 31 -1 Watts Batli Sodium salt of 5. 0 4. 0 140 40 None 1s, 900 (Bright) saccharin 3F-2 Methyl butynol- 0. 05 4.0 140 40 Pyruvic Acid 0. 97 0 ethylene oxide adduct 1 Ingredients of Watts bath are given in Table II.

Example 4 Example 5 The eifect of variations in the acidity, temperature, and cathode current density of a bright nickel Watts bath containing a stress-reducing additive on the internal stress The effect of the addition of various a-keto acids to a nickel sulfamate bath or to a bright nickel Watts bath on the internal stress of the nickel deposit is shown in of nickel deposits is shown in Table IV. The bath used Table V.

TABLE V Ex. Ingredients of Coneen- Tempera- Cathode Concen- Internal N 0. Type of Bath Bath tration, pH ture, F. Current Additive tration, Stress, g./l. Density g./l. p.s.i.

5A-1 Nickel Nl(SO3NHg)g 330 4.0 120 40 None 3, 000

sulfamate Boric Acid 30 5A-2 4.0 120 40 a-Ketobutyrie acid 1. 0 0 5.4-3 4. 0 120 40 Sodium 1. 5 0

Ketomalonate 5A-4 3.1 120 40 Z-Ketoglutaric acid 1. 0 0 511-5 3. 1 120 40 a-Ketosuccinic acid 1.0 0 5A-6 3. 1 120 40 a-Ketoadipic acid 1. 0 0 5B-1 Bright NiSOi-GH O 800 4.0 140 40 None -12, 000

nickel NiC12-6H2O 5B-2 Watts Boric Acid 37. 5 4. 0 140 40 a-Ketobutyric acid 2. 0 0

Bath Sodium salt of sac- 2. 4 charin 5B-3 Methyl butynol- 0. 065 4. 0 140 40 Sodium 4. 0 0 ethylene oxide Ketomalonate adduct I in each case contained 300 g. of NiSO '6H O, 60 g. of NiCl -6H O, 37.5 g. of boric acid, 2.4 g. of the sodium salt of saccharin, 0.065 g. of methyl butynol-ethylene oxide adduct, and 0.75 g. of pyruvic acid per liter. From the data in this table it will be seen that bright stressfree nickel deposits were obtained over a wide range of operating conditions.

sisting of a-keto carboxylic acids having from 3 to 10 carbon atoms and the water-soluble salts, esters, and amides of said acids.

2. An electroplating bath for producing stress-free electrodeposits of a metal selected from the group consisting of nickel, cobalt, zinc, and copper, comprising an aqueous solution of at least one water-soluble salt of the metal to be electrodeposited and approximately 0.25 g. to 115 g. per liter of said bath of a stress-reducing additive selected from the group consisting of a-keto carboxylic acids having from 3 to 10 carbon atoms and the water-soluble salts, esters, and amides of said acids.

3. An electroplating bath according to claim 2 wherein the quantity of the stress-reducing additive is 0.5 g. to 10 g. per liter.

4. An electroplating bath for producing stress-free nickel electrodeposits comprising an aqueous solution of at least one water-soluble salt of nickel and approximately 0.25 g. to 15 g. per liter of said bath of a stress-reducing additive selected from the group consisting of a-keto carboXylic acids having from 3 to 10 carbon atoms and the water-soluble salts, esters, and amides of said acids.

5. An electroplating bath for producing stress-free nickel electrodeposits comprising an aqueous solution of at least one inorganic salt of nickel and 0.5 g. to 10 g. per liter of said bath of a stress-reducing additive selected from the group consisting of u-keto carboxylic acids having from 3 to 10 carbon atoms and the water-soluble salts, esters, and amides of said acids.

6. An electroplating bath according to claim 5 wherein the stress-reducing additive is pyruvic acid.

7. An electroplating bath according to claim 5 wherein the stress-reducing additive is sodium pyruvate.

8. An electroplating bath according to claim 5 wherein the stress-reducing additive is ammonium pyruvate.

9. An electroplating plate according to claim 5 wherein the stress-reducing additive is 2-ketoglutaric acid.

10. An electroplating bath according to claim 5 wherein the stress-reducing additive is a-ketobutyric acid.

11. An electroplating bath according to claim 5 wherein the stress-reducing additive is sodium ketomalonate.

12. A method of producing a stress-free electrodeposit of a metal selected from the group consisting of nickel, cobalt, zinc, and copper, which comprises electrodepositing said metal from an electroplating bath comprising an aqueous solution of at least one water-soluble salt of the metal to be deposited and approximately 0.25 g. to 15 g. per liter of said bath of a stress-reducing additive selected from the group consisting of a-keto carboxylic acids having from 3 to 10 carbon atoms and the water-soluble salts, esters, and amides of said acids.

13. A method of producing a stress-free nickel electrodeposit which comprises electrodepositing nickel from an electroplating bath comprising an aqueous solution of at least one water-soluble salt of nickel and 0.5 g. to 10 g. per liter of said bath of a stress-reducing agent selected from the group consisting of a-keto carboxylic acids having from 3 to 10 carbon atoms and the water-soluble salts, esters, and amides of said acids.

14. The method of claim 13 wherein the stress-reducing additive is pyruvic acid.

15. The method of claim 13 wherein the stress-reducing additive is sodium pyruvate.

16. The method of claim 13 wherein the stress-reducing additive is 2-ketoglutaric acid.

17. The method of claim 13 wherein the stress-reducing additive is oc-kG'LOblltYIiC acid.

' 18. The method of claim 13 wherein the stress-reducing additive is sodium ketomalonate.

References Cited UNITED STATES PATENTS 2,497,725 2/1950 Griffith 204- JOHN H. MACK, Primary Examiner. G. KAPLAN, Assistant Examiner. 

1. AN ELECTROPLATING BATH FOR PRODUCING STRESS-FREE METAL ELECTRODEPOSITS OF A METAL SELECTED FROM THE GROUP CONSISTING OF NICKEL, COBALT, ZINC AND COPPER, COMPRISING AN AQUEOUS SOLUTION OF AT LEAST ONE WATER-SOLUBLE SALT OF THE METAL TO BE ELECTRODEPOSITED AND AN EFFECTIVE AMOUNT OF A STRESS-REDUCING ADDITIVE SELECTED FROM THE GROUP CONSISTING OF A-KETO CARBOXYLIC ACIDS HAVING FROM 3 TO 10 CARBON ATOMS AND THE WATER-SOLUBLE SALTS, ESTERS, AND AMIDES OF SAID ACIDS. 